object parented to bone does not stick to its place
Your rig uses weight paint for moving the body parts, whereas your glasses are curves with empty parent, which is parented to the bone - head.
Rotating bones causes them to affect the body with the weight used from weight paint, and the decorations are just parented to them, so body is deformed with weight (more or less), and decorations are just copying full rotation from it's parent.

So, when you rotate head you are affeting the neck as well, so weights are applied. However, your accesories use only head bone transform info.